The subject invention is primarily suited for use with alkaline type cells. Standard alkaline cells are conventionally manufactured with a cathode which generally comprises predominantly an oxide depolarizer, such as manganese dioxide usually admixed with a binder and conductive material such as graphite and the like. The cathode is generally pressed firmly against the inside wall of a container with the inner portion of the cathode left hollow so that the cathode assumes a closed end cylindrical structure with a centrally defined cavity. Separator material, usually formed into thin sheets of paper like material, is inserted into the cathode's central cavity and conforms to the cavity of the cathode. An anode which usually comprises a consumable anodic material such as powder zinc admixed with a gelling agent such as polyacrylic acid or carboxymethyl cellulose and a suitable alkaline electrolyte such as an aqueous potassium hydroxide solution, is then extruded into the cavity of the separator. Thus the sheets of separator material electrically isolates the anode from the cathode while simultaneously permitting ions to flow between the electrodes. Generally, the separator strips can be forced downward and through the cathode's central cavity so that the separator's center region is parallel with and close to the bottom of the container. Occasionally, the insertion of the separator, using a rod driving means, forces the center region of the separator into the cathode material at the bottom of the container. On occasions, small pieces of the cathode are driven into and partially through the separator. This can result in short circuit of the anode and cathode of the cell thus rendering the cell useless. In addition, sometimes during discharge of the cell, zinc dendrites are formed that can extend through the separator at the bottom region of the container and short circuit the cell. These problems are difficult to detect because the cell would have to be disassembled in order to inspect the bottom central portion of the separator for evidence of cathode mix penetration or dendrite penetration.
British Patent 2,181,584 describes a method of producing a separator basket for standard alkaline batteries. This process uses a mandrel to form and support a tube of separator material on which the separator material is folded over one end of the tube and then a small amount of sealant is dispensed on the outside surface of the folded over bottom of the separator to form a "basket". The sealant seals the end of the separator basket and in some applications may secure the separator to other cell components such as the cathode. A final step may include insertion of a can containing a molded cathode over the mandrel-supported separator.
Canadian Patent 993,934 discloses that carbon-zinc cells are commonly manufactured by inserting a carbon rod into a zinc can that contains cathode mix. This manufacturing process is known to suffer from occasional mix penetration through the separator beneath the carbon rod. This patent describes bottom cups that have a mix impermeable substance applied to the central region of the cup's inner surface. Coating only the central portion of the bottom cup effectively prevents mix penetration directly beneath the carbon rod while maximizing both (1) the cup's surface area that is available for soak up of the electrolyte and (2) the anode to cathode surface area.
U.S. Pat. No. 3,748,181 describes the concept of attaching a plastic strip to the top edge of a separator that is formed into a convoluted wound separator basket for cylindrical batteries. The film eliminates any internal short circuits that are caused by mix smearing near the open end of the can during the mix insertion process.
U.S. Pat. No. 4,220,693 describes a separator for alkaline cells. One of the embodiments discloses polypropylene secured to the entire surface of the base separator film.
U.S. Pat. No. 3,097,975 describes internal short circuits that are caused when one electrode "grows" through the separator's interstices. The suggested solution is to use a pulverulent material, such as aluminum oxide or magnesium oxide, to reduce the size of the interstices.
It is an object of the present invention to provide a means for effectively preventing internal shorting at the bottom area of a cell's container.
It is another object of the present invention to provide a separator for alkaline cells that is easy and economical to make and that can effectively prevent the cathode material of the cell from penetrating through the separator at the bottom area of the container of the cell.
It is another object of the present invention to provide an electrical insulating barrier layer or film on the bottom surface of the separator between the anode and cathode.
It is another object of the invention to provide a process for assembling the separator into the cell.
These and other objects and advantages of the invention will be apparent from the following description and drawings.